Legends abound regarding the first meeting of Spencer Tracy and Katharine Hepburn, the screen icons whose 26-year extramarital relationship would become the talk of Hollywood.

And in James Curtis's exhaustive and insightful new biography we get the definitive account of this and practically every other incident in Tracy's life.

Hepburn and Tracy met for the first time on the MGM backlots on a hot August morning in 1941. They were in the company of writer/director Joe Mankiewicz, and Hepburn was hoping to persuade Tracy to co-star with her in Woman of the Year.

Tracy's publicists claimed he was 5ft 10in but he was probably closer to 5ft 7in, and the statuesque Hepburn's opening remark was: "Sorry I've got these high heels on -- when we do the movie I'll be careful about what I wear." Tracy glowered and said nothing, so it was left to Mankiewicz to dryly comment: "Don't worry, Kate, he'll cut you down to size."

Thus began one of the most famously tortured romances in movie history, and Tracy and Hepburn's turbulent association provides many of the best stories in Curtis's meticulously researched biography.

But the author is careful not to let the bombastic Hepburn dominate his book, and devotes at least the first half of it to the other big woman in Tracy's life -- his wife of 44 years, Louise Treadwell.

This approach is scrupulously fair but problematic, because although a lot more competently written than the general run of film biographies, Curtis's book only really gets going when Hepburn bursts on to the stage.

Tracy was regarded as the finest screen actor of his generation, was lauded by everyone from John Ford to Humphrey Bogart and won two best actor Oscars but might have won 10.

Solidly Irish Catholic on his father's side, Tracy was born in 1900 in Milwaukee, Wisconsin, a blue-collar Midwestern town famous for "beer and bratwurst".

By all accounts he was a little tearaway -- a tough, pig-headed, freckle-faced urchin who was always getting himself into trouble. He was a poor student, too, and even the nuns and Jesuits at his schools failed to knock much sense into him.

He was toying with the notion of studying medicine at Ripon College in the early 1920s when he stumbled on his true calling. Given a role in a melodrama called The Truth, he revealed a remarkable stage charisma and innate acting instinct.

"Acting to me is always reacting," Tracy liked to say, and his famous stillness and intensity got him noticed first on Broadway, then in Hollywood. Despite his rather heavy Irish features, he became a big star in the 1930s thanks to films like Boys Town and Captains Courageous.

He'd met Louise Treadwell while working for a travelling theatre company in the early 1920s.

The pair married and had two children, but when his son John turned out to be almost totally deaf, Tracy was consumed by Catholic guilt and decided the affliction was his fault.

For Tracy had not been the ideal husband: a strong streak of alcoholism ran through his family on his father's side, and Tracy was a messy, violent drinker who'd sometimes binge for weeks on end. There were numerous lost weekends when he would eventually be found by the studio in a cheap hotel with bottles and prostitutes. He also had an eye for his leading ladies, and is said to have had affairs with Loretta Young, Joan Crawford, Gene Tierney and Ingrid Bergman. It was Hepburn, however, who really stole his heart.

The pair fell in love on the set of Woman of the Year, the first of nine hit films they would make together. In that movie they played a married couple who battle for supremacy, and the storyline echoed their relationship off-screen.

"I found him irresistible," Hepburn said later, "I would have done anything for him."

But his devotion to her did not extend to leaving his wife, a prospect he could never contemplate.

So for the next 26 years Tracy lived a kind of double life, with a public marriage that was something of a sham and a private relationship that was a marriage in all but name. It was a difficult and at times explosively turbulent partnership, but it was Hepburn who cared for Tracy when he became gravely ill during the making of his last film, Guess Who's Coming to Dinner.

After his death, in June of 1967, Hepburn had the tact to stay away from Tracy's funeral.

But afterwards she phoned Louise to try to make amends. She even suggested they could now be friends, saying "you knew him at the beginning, I at the end".